The invention relates to a projection display device comprising an illumination system having a light source and an optical system for providing an illumination beam, an image display system for modulating the illumination beam with image information and for projecting an image on a screen, filter means for transmitting a first portion of radiation from the light source having a first wavelength range in the visible area to the optical system, and for absorbing a second portion of the radiation from the light source, the second portion having a second wavelength range outside the visible area, and cooling means comprising a liquid for cooling the filter means.
Projection display devices may be used in both rear and front image projecting systems. In a rear projection system, the projection display device projects an image representing television or datagraphic information on the rear side of a diffusing transparent screen, which front side is directed to a viewing audience. In a front projecting system, the projection display device projects an image representing television or datagraphic information on the front side of a reflecting screen, which front side is directed to the viewing audience.
Such a projection display device is known from Japanese Kokai 09-005734. The known projection display device comprises an illumination system for providing an illumination beam and an image display system for modulating an illumination beam to be supplied by the illumination system with image information and projecting an image on a screen. The illumination system comprises a light source and an optical system for forming the illumination beam. The light source may be an ultra-high pressure discharge lamp. Besides visible radiation this kind of lamp produces, also ultra-violet radiation with wavelengths mainly in the range between 250 and 425 nm and infra-red radiation with wavelengths mainly in the range between 800 and 950 nm. In order to filter the unwanted portions of the radiation from the light source, such as the ultra-violet and infra-red radiation, an ultra-violet absorbent filter and an infra-red absorbent filter may be positioned in the light path between the light source and the optical system. In order to keep the ultra-violet and infra-red absorbent filters below their maximum operating temperature, a cooling container comprising two parallel plates containing a cooling liquid is present, which container comprises a transparent portion for a wavelength range in the visible area for passing the illumination beam. This cooling means is positioned in the illumination beam and attached to the ultra-violet absorbent filter or the infra-red absorbent filter to transport the heat generated in the ultra-violet absorbent filters or the infra-red filter, respectively, to the environment of the projection display device.
A disadvantage of the projection display device is that the thermal conductance between the ultra-violet absorbent filter and the container is limited and the operating temperature of the filters can be exceeded, which affects the cut-off wavelengths and the lifetime of the filters.
It is an object of the invention to provide a projection display device having an extended lifetime. This object can be achieved by the projection display device in accordance with the invention, and is characterized in that the filter means comprises the liquid which contains a radiation-absorbent additive for absorbing the second range of wavelengths. This invention is based on the recognition that the heat generated by the absorption of the radiation can be directly transported to the environment by convection in the liquid whereas in the conventional system the heat transport is limited by conduction in the ultra-violet or infra-red absorbent materials. In the new projection display device, heat transport via convection is more efficient so that the filter means can be operated at a lower operating temperature, which extends the lifetime.
A further advantage is that the stability of the cut-off wavelength of the filter means is improved because of a temperature dependency of the cut-off wavelength. A more constant temperature provides a more stable cut-off wavelength of the filter means. Furthermore, the improved cooling may allow a more compact design of the projection display device.
A particularly advantageous embodiment of the projection display according to the invention is characterized in that the liquid comprises a solvent and the radiation-absorbent additive is a substance which is soluble in the solvent for absorbing radiation having the second range of wavelengths. The second range of wavelengths may be an ultra-violet range between 250 and 425 nm or an infra-red range between 800 and 950 nm.
A further embodiment of the projection display device is characterized in that the solvent is water and the radiation-absorbent substance is one of the group of benzophenonephosphates and benzotriazolephosphates. Also benzophenonesulphates or benzotriazolesulphates can be used as radiation-absorbent substances, both of which are also soluble in water. These radiation-absorbent substances mainly absorb ultra-violet radiation in a range between 350 and 425 nm.
A further embodiment of the projection display device is characterized in that the solvent is an organic liquid and the radiation-absorbent additive is one of the group of benzophenones and benzotriazoles. This radiation-absorbent substance also absorbs ultraviolet radiation in a range between 250 and 425 nm.
A further embodiment of the projection display device is characterized in that the solvent is water and the radiation-absorbent substance is one of the group of carbocyanines. This radiation-absorbent substance absorbs mainly infra-red radiation in a wavelength range between 800 and 950 nm.